1. Field of the Invention
The present invention generally relates to an image pickup apparatus, and more particularly, it relates to an auto-focusing apparatus of the image pickup apparatus. The present invention is suitable for, for example, an auto-focusing apparatus and an image pickup apparatus performing optimal auto-focus control in accordance with a format of a recording signal.
2. Description of the Related Art
In recent years, the main stream of an auto-focus (AF) apparatus of a video camera is of a type in which a sharpness of a screen is detected from an image signal obtained by photoelectrically converting an object image with an image pickup device (e.g., a CCD) and so forth, so as to serve as an AF evaluation value, and focus adjustment is performed by controlling the position of a focus lens so as to achieve a maximum AF evaluation value.
A level of a high frequency component of an image signal extracted by a band-pass filter is generally used as an AF evaluation value. In other words, as shown in FIG. 11, when a normal object is photographed, the more the object is brought into focus (the closer the object comes to a focal point), an AF evaluation value becomes greater. As such, a point making the level maximum thereat is defined as a focal position. Meanwhile, it is known that an AF evaluation value has a characteristic that making the center frequency of the band-pass filter higher relative to the frequency band of an image signal leads to a graph of the AF evaluation value having a sharp shape, and making the same lower leads it having a gentle shape. FIG. 11 is a graph of the relationships between position of a focus lens and AF evaluation value of each of the high and low frequency components of an image signal, wherein the vertical and horizontal axes respectively indicate an AF evaluation value and a position of the focus lens value.
The frequency characteristic of the band-pass filter extracting an AF evaluation value is set such that a sufficient variation (reduction) in the AF evaluation value is obtained by moving the focus lens in the neighborhood of its focal point and within its focal depth, so as to clearly notice the peak of the AF evaluation value, and also that, even when the focus lens moves in a direction away from the focal point, a clear difference of the AF evaluation value can be obtained with movement of the focus lens of the order of its focal depth.
Meanwhile, with respect to recording types, other that a known type of standard definition (SD) format (720 H×480 V), a type of a high definition (HD) format (1440 H×1080 V) higher than the known SD type is proposed. Corresponding to such formats, a video camera generating an image signal with the HD format from all area of the CCD and another image signal with the SD format from a part of the area of the CCD by applying a reduction process on it, and recording these signals is proposed. For such a video camera having image-taking modes of mutually different recording types as described above, a method for generating an AF evaluation value by applying the common process on an image signal with the HD format, and controlling the position of the focus lens so as to adjust the focal point. (to perform AF control) is proposed (see Japanese Patent Laid-Open No. 7-107359, for example)
With the method proposed in the above-mentioned patent document, regardless of recording formats of an image signal, an AF evaluation value is generated by applying the common process on the image signal so as to perform the AF control. Accordingly, the band-pass filter having a frequency characteristic adjusted for the HD format is set such that, even when the focus lens is driven in the neighborhood of the focal point by an amount of a focal depth in accordance with the HD format, a change (decrease) in the AF evaluation value is obtained so as to notice its peak. Corresponding to the above setting, determining values of a focus move quantity, a change in the evaluation value, and the like are set for performing the AF control.
Unfortunately, since an image is displayed in a reduced manner in accordance with the SD format, a focus move quantity for noticing a blue on a screen is greater than that with the HD format. Accordingly, with the same focus move quantity, a change in blur on a display screen of an image signal is less noticeable with the SD format than with the HD format, thereby causing a photographer more difficult to notice movement of AF. In other words, when an AF operation in accordance with the SD format is performed in the same fashion as performed in accordance with the HD format, availability for coping with the AF is considerably deteriorated.
When the generation process of an AF evaluation value with the SD format is arranged for the HD format, the band-pass filter has a high center frequency, and the AF evaluation value has a sharper characteristic shape than that with the known SD format. Hence, it is difficult to determine a direction of the focal point at occurrence of a large blur, thereby resulting in a poorer AF performance than those of other models exclusive for the SD format.
Whereas, when the band-pass filter having a frequency characteristic arranged for the SD format is used, the AF evaluation value has a broader shape with respect to the position of the focus lens, whereby the focal point is not clearly noticed even when the focus lens is driven in the neighborhood of the focal point, within the focal depth in accordance with the HD format.
Although AF evaluation values optimal respectively for the HD and SD formats are needed, no image pickup apparatus and/or no AF apparatus generating an optimal AF evaluation value for itself in accordance with a format type is heretofore present.